Esters of polyhydric alcohols, also called polyol esters, find a variety of uses on a large scale in industry, for example as plasticizers or lubricants. The selection of suitable starting materials allows the physical properties, for example boiling point or viscosity, to be controlled, and the chemical properties, such as hydrolysis resistance or stability to oxidative degradation, to be taken into account. Polyol esters can also be tailored to the solution of specific performance problems. Detailed overviews of the use of polyol esters can be found, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, 1985, VCH Verlagsgesellschaft, Vol. A1, pages 305-319; 1990, Vol. A15, pages 438-440, or in Kirk Othmer, Encyclopedia of Chemical Technology, 3rd edition, John Wiley & Sons, 1978, Vol. 1, pages 778-787; 1981, Vol. 14, pages 496-498.
The use of polyol esters as lubricants is of great industrial significance, and they are used particularly for those fields of use in which mineral oil-based lubricants meet the requirements set only incompletely. Polyol esters are used especially as turbine engine and instrument oils. Polyol esters for lubricant applications are based frequently on 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,2-hexanediol, 1,6-hexanediol, neopentyl glycol, trimethylolpropane, pentaerythritol, 2,2,4-trimethylpentane-1,3-diol, glycerol or 3(4),8(9)-dihydroxymethyltricyclo[5.2.1.02,6]decane, also known as TCD alcohol DM, as the alcohol component.
Polyol esters are also used to a considerable degree as plasticizers. Plasticizers find a variety of uses in plastics, coating materials, sealing materials and rubber articles. They interact physically with high-polymer thermoplastic substances, without reacting chemically, preferably by virtue of their swelling and dissolution capacity. This forms a homogeneous system, the thermoplastic range of which is shifted to lower temperatures compared to the original polymers, one result being that the mechanical properties thereof are optimized, for example deformation capacity, elasticity and strength are increased, and hardness is reduced.
In order to open up the widest possible fields of use to plasticizers, they must fulfil a series of criteria. They should ideally be odourless, colourless, and light-, cold- and heat-resistant. Moreover, it is expected that they are insensitive to water, comparatively nonflammable and not very volatile, and are not harmful to health. Furthermore, the production of the plasticizers should be simple and, in order to meet ecological requirements, avoid waste substances, such as by-products which cannot be utilized further and wastewaters comprising pollutants. A specific class of polyol esters (they are referred to as G esters for short) contains diols or ether diols as the alcohol component, for example ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, 1,2-propylene glycol or higher propylene glycols. They can be prepared in different ways. In addition to the reaction of alcohol and acid, optionally in the presence of acidic catalysts, further processes are employed in practice to obtain G esters, including the reaction of diol with acid halide, the transesterification of a carboxylic ester with a diol, and the addition of ethylene oxide onto carboxylic acids (ethoxylation). In industrial manufacture, only the direct reaction of diol and carboxylic acid and the ethoxylation of carboxylic acids have become established as production processes, preference usually being given to the esterification of diol and acid. This is because this process can be performed with no particular complexity in conventional chemical apparatus, and it affords chemically homogeneous products. Compared to this, ethoxylation requires extensive and costly technical equipment.
The direct esterification of alcohols with carboxylic acids is one of the basic operations in organic chemistry. In order to increase the reaction rate, the conversion is typically performed in the presence of catalysts. The use of one of the reactants in excess and/or the removal of the water formed in the course of the reaction ensures that the equilibrium is shifted in accordance with the law of mass action to the side of the reaction product, i.e. of the ester, which means that high yields are achieved.
Comprehensive information regarding the preparation of esters of polyhydric alcohols, also including esters of ethylene glycols and fatty acids, and regarding the properties of selected representatives of these compound classes can be found in Goldsmith, Polyhydric Alcohol Esters of Fatty Acids, Chem. Rev. 33, 257 ff. (1943). For example, esters of diethylene glycol, of triethylene glycol and of polyethylene glycols are prepared at temperatures of 130 to 230° C. over reaction times of 2.5 to 8 hours. To remove the water of reaction, carbon dioxide is used. Suitable catalysts mentioned for the esterification of polyhydric alcohols are inorganic acids, acidic salts, organic sulphonic acids, acetyl chloride, metals or amphoteric metal oxides. The water of reaction is removed with the aid of an entraining agent, for example toluene or xylene, or by introducing inert gases such as carbon dioxide or nitrogen.
The production and the properties of fatty acid esters of the polyethylene glycols are discussed by Johnson (edit.), Fatty Acids in Industry (1989) Chapter 9, Polyoxyethylene Esters of Fatty Acids, and a series of preparative hints are given. Higher diester concentrations are achieved by the increase in the molar ratio of carboxylic acid to glycol. Suitable measures for removing the water of reaction are azeotropic distillation in the presence of a water-immiscible solvent, heating while passing through an inert gas, or performing the reaction under reduced pressure in the presence of a desiccant. When the addition of catalysts is dispensed with, longer reaction times and higher reaction temperatures are required. Both reaction conditions can be made milder by the use of catalysts. In addition to sulphuric acid, organic acids such as p-toluenesulphonic acid and cation exchangers of the polystyrene type are the preferred catalysts. The use of metal powders, such as tin or iron, is also described. According to the teaching from U.S. Pat. No. 2,628,249, colour problems in the case of catalysis with sulphuric acid or sulphonic acid can be alleviated when working in the presence of activated carbon.
As further metallic catalysts, titanium alkoxides, carboxylates or chelates, zirconium alkoxides, carboxylates or chelates or tin alkoxides, carboxylates or chelates are also used for preparation of polyol esters, for example according to U.S. Pat. No. 5,324,853 A1. Such metal catalysts can be regarded as high-temperature catalysts, since they do not reach their full activity until high esterification temperatures, generally above 180° C. They are frequently added not at the start of the esterification reaction but after the reaction mixture has already been heated up and has partly reacted with elimination of water. In spite of the higher reaction temperatures and longer reaction times required compared to conventional sulphuric acid catalysis, catalysis with such metal compounds affords crude esters having a comparatively low colour number. Commonly used esterification catalysts are, for example, tetraisopropyl orthotitanate, tetrabutyl orthotitanate, tetrabutyl zirconate or tin(II) 2-ethylhexanoate.
In the catalytic esterification reaction of polyols with carboxylic acids, based on the component present in deficiency, a high conversion is attained within a comparatively short time, but a comparatively long reaction time has to be accepted for the residual conversion to the desired polyol esters. In that case, a polyol ester having an acceptable residual content of partly esterified products is obtained, expressed by the hydroxyl number in mg KOH/g (according to DIN 53240) or by the content of partially esterified products determined by gas chromatography, but long reaction times are economically disadvantageous since they restrict the performance of the industrial production plant. In order also to accelerate the residual conversion, U.S. Pat. No. 5,324,853 A1 proposes vigorous mixing of the reaction mixture.
After the esterification reaction has ended, sufficient removal of the metal catalyst should be ensured, since metal traces in the purified polyol esters can impair the use thereof as plasticizers or lubricants, for example by influencing the electrical conductivity or the stability to atmospheric oxygen. According to the mode of operation from U.S. Pat. No. 5,324,853 A1, the crude esterification mixture is admixed with an aqueous soda solution and optionally with activated carbon. This mode of operation hydrolyses the metal compounds to insoluble solids, and they can be filtered off prior to the further workup of the crude ester compound. According to U.S. Pat. No. 4,304,925 A1, the crude esterification product, prior to addition of alkali, is first admixed with water and heat-treated. This converts the hydrolysed metal compounds to precipitates of good filterability.
According to WO 2011/042116 A1, the reaction of polyols with linear or branched aliphatic monocarboxylic acids having 3 to 20 carbon atoms is effected in the presence of a Lewis acid and in the presence of an adsorbent, followed by performance of a steam treatment. The steam treatment destroys catalyst residues still present and converts them to hydrolysis products of good filterability. The adsorbent already present during the esterification reaction facilitates the separation of the catalyst conversion products.
EP 2 308 823 A2 likewise relates to a process for preparing polyol esters by reacting polyols with linear or branched aliphatic monocarboxylic acids having 3 to 20 carbon atoms in the presence of an adsorbent. The polyol ester obtained can be subjected to another aftertreatment with an adsorbent.
EP 0 439 722 A1 concerns a process for workup of a crude ester mixture from the titanium-catalyzed reaction of primary C6 to C14 alcohols with di- and tricarboxylic acids or anhydrides thereof. After removing the unconverted alcohol, activated carbon is added and the residual alcohol is removed by treatment with steam.
WO 2005/021482 A1 also describes an esterification process in which the esterification reaction is followed by workup of the crude product by base treatment, steam treatment, filtration and another stripping operation. There follows a treatment with an adsorbent and subsequently the filtration of the adsorbent, optionally in the presence of a filtering aid. The adsorbent used is preferably activated carbon, which can also be used together with the filtering aid in the workup of the crude ester. Both the activated carbon and the filtering aid should each have a pH of 6 to 11.
The prior art regarding preparation of polyol esters under metal catalysis entails either a particular reactor design in order to complete the esterification reaction within an economically acceptable time or an additional treatment with water under hot conditions, for example also in the form of a steam treatment, in order to very substantially remove the metallic catalyst after the esterification reaction has ended to form hydrolysis products of good filterability.
The problem addressed was therefore that of improving the known processes and optimizing the process by balancing and simplifying the successive component steps in the overall process and simplifying the recovery of polyol esters in high quality, such that polyol esters can have maximum versatility of use.